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AbstractBackground and AimsLiver‐related clinical consequences of non‐alcoholic fatty liver disease (NAFLD) are seen only in the minority of patients with advanced fibrosis. The aim of our study was to generate insight into a potential endocrine basis of steatohepatitis with advanced fibrosis in NAFLD.MethodsBiopsy and blood samples were prospectively collected from patients with medically complicated obesity. Patients were categorized, according to liver histology, into: (i) normal, (ii) simple steatosis (SS), (iii) non‐alcoholic steatohepatitis (NASH) with fibrosis stage (FS) 0–1 and (iv) NASH with FS ≥ 2. A broad panel of potential biomarkers included DHEA‐S, growth hormone (GH), homeostasis model assessment‐insulin resistance (HOMA‐IR), leptin, resistin, adiponectin and cytokeratin 18 (CK‐18) fragments.ResultsWe studied 160 patients (mean BMI 46.8 ± 8.2 kg/m2). Liver biopsies demonstrated normal histology in 10%, SS in 45%, NASH with FS 0–1 in 37.5% and NASH with FS ≥ 2 in 7.5%. C‐reactive protein, IL‐6, GH, CK‐18, adiponectin, HOMA‐IR and quantitative insulin sensitivity check index (QUICKI) were significantly associated with NASH in univariate analysis, but overall predictivity of these parameters was low (AUC ROC = 0.62–0.68). In contrast, all patients with NASH with FS ≥ 2 had insulin resistance, as measured by QUICKI, and GH levels <0.45 ng/ml and all but one patient with NASH FS 2–3 had low DHEA levels (<123 μg/dl).ConclusionsLow serum levels of GH and DHEA are very common in patients with NASH with more advanced fibrosis. Other biomarkers, including CK‐18 fragment levels, have predictivity characteristics that would be of low clinical utility for distinguishing patients with normal histology or SS from those with NASH. These findings demonstrate an endocrine profile associated with advanced fibrosis.

Although there are small animal platforms that recapitulate some of the histological features of nonalcoholic fatty liver disease, there are no small animal models of nonalcoholic steatohepatitis (NASH) with consistent hepatocellular ballooning and progressive fibrosis that also exhibit fidelity to the human condition physiologically. We examined the metabolic and histological effects of a diet on the basis of the composition of “fast food” (high saturated fats, cholesterol, and fructose). Mice ( n = 8 in each group) were assigned to diets as follows: 1) standard chow (SC), i.e., 13% energy as fat [1% saturated fatty acids (SFA)], 2) high fat (HF), i.e., 60% energy as fat (1% SFA), and 3) fast food (FF), i.e., 40% energy as fat (12% SFA, 2% cholesterol). All three diets were supplemented with high fructose. All diets produced obesity. The HF and FF diets produced insulin resistance. Liver histology was normal in animals fed the SC diet. Steatohepatitis with pronounced ballooning and progressive fibrosis (stage 2) was observed in mice fed the FF diet. Although the HF diet produced obesity, insulin resistance, and some steatosis; inflammation was minimal, and there was no increase in fibrosis. The FF diet produced a gene expression signature of increased fibrosis, inflammation, and endoplasmic reticulum stress and lipoapoptosis. A diet based on high cholesterol, high saturated fat, and high fructose recapitulates features of the metabolic syndrome and NASH with progressive fibrosis. This represents a novel small animal model of fibrosing NASH with high fidelity to the human condition. These results highlight the contribution of dietary composition to the development of nonalcoholic fatty liver disease and NASH.

Insulin’s effect on the synthesis of liver proteins remains to be fully defined. Previous studies using various surrogate measures of amino acyl–tRNA have reported variable results of insulin’s effect on liver protein synthesis. We determined the effect of insulin with or without amino acid supplementation on the synthesis rates of liver proteins (tissue, albumin, and fibrinogen) using l-[1-13C]Leu as a tracer in 24 male miniature swine. In addition, we compared the isotopic enrichment of different precursors of liver proteins with that of amino acyl−tRNA using l-[1-13C]Leu and l-[15N]Phe as tracers. Although liver tissue fluid enrichment of [13C]Leu and [15N]Phe and that of plasma [13C]ketoisocaproatic acid (KIC) were very similar to that of tRNA, plasma isotopic enrichment of both Leu and Phe were substantially higher (P &lt; 0.01) and VLDL apolipoprotein-B100 enrichment was lower (P &lt; 0.01) than the respective amino acyl−tRNA enrichment. Plasma KIC enrichment most accurately predicted leucyl-tRNA enrichment, whereas plasma Leu enrichment was best correlated with that of tRNA. Neither insulin alone nor insulin plus amino acid infusion had an effect on liver tissue protein synthesis. In contrast, insulin alone decreased the albumin synthesis rate, and insulin with amino acids maintained the albumin synthesis rate. Insulin with or without amino acids inhibited the fibrinogen synthesis rate. These results, based on synthetic rates using amino acyl–tRNA, were consistent with those obtained using KIC or tissue fluid Leu or Phe as precursor pools. These studies demonstrated that plasma KIC enrichment is a convenient and reliable surrogate measure of leucyl-tRNA in liver. We also concluded that insulin has differential effects on the synthesis rates of liver proteins. Whereas insulin with or without amino acid supplement has no acute effect on the synthesis of liver tissue protein, insulin has a substantial inhibitory effect on fibrinogen synthesis. In contrast, insulin administration along with amino supplement is necessary to maintain albumin synthesis rate.

In vivo studies have reported conflicting effects of insulin on mixed tissue protein synthesis rates. To test the hypothesis that insulin has differential effects on synthesis rates of various protein fractions in different organs, we infused miniature swine (n = 8 per group) with saline, insulin alone (at 0.7 mU/kg−1 · min−1), or insulin plus an amino acid mixture for 8 h. Fractional synthesis rate (FSR) of mitochondrial and cytoplasmic proteins in liver, heart, and skeletal muscle, as well as myosin heavy chain (MHC) in muscle, were measured using l-[1-13C]leucine as a tracer. The FSR of mitochondrial and cytoplasmic proteins were highest in liver, followed by heart and then muscle. Mitochondrial FSR in muscle was higher during insulin and insulin plus amino acid infusions than during saline. Insulin had no significant effect on FSR of MHC in muscle. In contrast, FSR of both mitochondrial and cytoplasmic proteins were not stimulated by insulin in liver. Insulin also did not increase FSR of mitochondrial in heart, whereas insulin and amino acid stimulated FSR of cytoplasmic protein. In conclusion, insulin stimulates the synthesis of muscle mitochondrial proteins, with no significant stimulatory effect on synthesis of sarcoplasmic and MHC. These results demonstrate that insulin has different effects on synthesis rates of specific protein fractions in the liver, heart, and skeletal muscle.

The catabolic state of poorly controlled type 1 diabetes has largely been attributed to insulin deficiency. However, the role of hyperglucagonemia, which occurs concomitantly with insulin deficiency, has not been fully investigated. We studied the effects of hyperglucagonemia during insulin deprivation on energy expenditure (using indirect calorimetry) and protein metabolism (using L-[1-(13)C,15N]leucine and L-[1-(13)C]leucine as tracers) in 12 type 1 diabetic subjects. Five protocols were used: insulin treatment, insulin deprivation, insulin deprivation with suppression of endogenous glucagon with somatostatin (SRIH) and growth hormone replacement, insulin deprivation with endogenous glucagon suppression with SRIH (no growth hormone replacement), and insulin deprivation with SRIH and a high level of glucagon replacement (no growth hormone replacement). It was observed that leucine oxidation and the resting metabolic rate (RMR) were significantly lower during insulin treatment and insulin deprivation with concomitant SRIH infusion (lowering glucagon) than during insulin deprivation alone. Replacement of glucagon at a high level during SRIH infusion in the insulin-deprived state increased leucine oxidation and the RMR. Hyperglucagonemia was also associated with a trend for decreased protein synthesis. Hyperglucagonemia did not affect leucine transamination. Insulin replacement decreased leucine flux and oxidation. Leucine oxidation (R2 = 0.79) and the RMR (R2 = 0.81) were seen, by multiple regression analysis, to correlate with glucagon levels and not with other hormones. We conclude that while insulin deficiency increases protein breakdown, hyperglucagonemia is primarily responsible for the increased leucine oxidation and RMR seen during insulin deprivation.